Evaluation of propofol for general anesthesia in premedicated horses

Objective - To evaluate selected hemodynamic, respiratory, and behavioral responses to propofol in horses premedicated with xylazine or detomidine. Design - Xylazine (0.5 and 1.0 mg/kg of body weight) was administered IV on different days to each of 6 horses prior to IV administration of propofol (2 mg/kg). In a second group of 6 horses, detomidine (15 and 30 μg/kg) was similarly studied. Animals - 2 groups of 6 mature healthy horses. Procedure - Rectal temperature, heart and respiratory rates, arterial blood gas tensions, and direct arterial blood pressures were recorded before and at fixed intervals after drug administration. Induction and recovery events were quantitatively and qualitatively assessed. Cardiopulmonary and behavioral data to follow were statistically analyzed (P ≤ 0.05). Results - Heart rate decreased in dose-dependent manner from a mean (± SD) of 39.5 ± 5.1 beats/min after xylazine and detomidine. Second-degree atrioventricular dissociation was commonly seen at the higher drug doses. After propofol administration, heart rate either transiently increased or was less depressed early in recumbency, compared with predrug values. Direct arterial blood pressures varied inconsistently from predrug values. Mean arterial carbon dioxide tension tended to increase after drug administration (significance variable) from predrug values of 42 to 46 mm of Hg in both drug groups. After xylazine or detomidine administration, arterial oxygen tension decreased significantly from predrug values of 97 to 103 mm of Hg. The magnitude and duration of decrease was dose-dependent and greatest during recumbency. Behavioral responses to anesthetic induction were variable, but horses were uniformly calm and coordinated during recovery. Recumbency time increased in reponse to the higher dose of either premedicant drug. Mean (± SD) times to standing were 25.02 ± 4.42 and 35.57 ± 6.83 minutes for the low and high doses of xylazine, respectively and 41.04 ± 11.21 and 52.64 ± 14.67 minutes for the low and high doses of detomidine, respectively. Conclusion - Neither xylazine nor detomidine prevented excitation associated with propofol injection in horses. Clinical Relevance - Xylazine- or detomidine-propofol combinations likely will not replace common anesthetic induction techniques for horses. However, recovery characteristics associated with propofol encourage further study in horses.